1. Field of the Invention
The present invention relates to a droplet ejector and an ink-jet printhead using the same. More particularly, the present invention relates to a droplet ejector that ejects ink droplets by expanding and contracting a volumetric structure sensitive to an external stimulus, and an ink-jet printhead using the same.
2. Description of the Related Art
Typically, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. Ink-jet printheads are largely categorized into two types depending on which ink droplet ejection mechanism is used. A first type is a thermally driven ink-jet printhead in which a heat source is employed to form and expand bubbles in ink causing ink droplets to be ejected. A second type is a piezoelectrically driven ink-jet printhead in which a piezoelectric material deforms to exert pressure on ink causing ink droplets to be ejected.
Hereinafter, the ink ejection mechanism in the thermally driven ink-jet printhead will be described in greater detail. When a pulse current flows through a heater formed of a resistance heating material, the heater generates heat and ink adjacent to the heater is instantaneously heated to about 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated, expand, and apply pressure to an interior of an ink chamber filled with ink. As a result, ink near a nozzle is ejected from the ink chamber in droplet form through the nozzle.
The thermal driving method includes a top-shooting method, a side-shooting method, and a back-shooting method depending on a growth direction of bubbles and an ejection direction of ink droplets.
The top-shooting method is a method in which the growth direction of bubbles is the same as the ejection direction of ink droplets. The side-shooting method is a method in which the growth direction of bubbles is perpendicular to the ejection direction of ink droplets. The back-shooting method is a method in which the growth direction of bubbles is opposite to the ejection direction of ink droplets.
FIG. 1 illustrates a cross-sectional view of a structure of a conventional thermally driven ink-jet printhead. Referring to FIG. 1, the thermally driven ink-jet printhead includes a base plate 30 formed by a plurality of material layers stacked on a substrate, a barrier layer 40 that is formed on the base plate 30 and defines an ink chamber 42, and a nozzle plate 50 stacked on the barrier layer 40. Ink fills the ink chamber 42, and a heater 33 that heats ink to generate bubbles in ink is installed under the ink chamber 42. Although FIG. 1 illustrates a single exemplary nozzle 52, a plurality of nozzles 52 through which ink is ejected may be formed in a position corresponding to each of a plurality of ink chambers 42.
The vertical structure of the ink-jet printhead described above will now be described in greater detail.
An insulating layer 32 formed of silicon is formed on a substrate 31 for providing insulation between a heater 33 and the substrate 31. The insulating layer 32 is formed by depositing a silicon oxide layer on the substrate 31. The heater 33, which heats ink in the ink chamber 42 to generate bubbles in ink, is formed on the insulating layer 32. The heater 33 is formed by depositing tantalum nitride (TaN) or thin-film tantalum-aluminum (TaAl) on the insulating layer 32 in a thin film shape. A conductor 34 for applying a current to the heater 33 is formed on the heater 33. The conductor 34 is made of a metallic material having good conductivity, such as aluminum (Al) or an aluminum (Al) alloy. Specifically, the conductor 34 is formed by depositing aluminum (Al) on the heater 33 to a predetermined thickness and patterning a deposited resultant in a predetermined shape.
A passivation layer 35 for passivating the heater 33 and the conductor 34 is formed on the heater 33 and the conductor 34. The passivation layer 35 prevents the heater 33 and the conductor 34 from oxidizing or directly contacting ink, and is formed by depositing silicon nitride. In addition, an anti-cavitation layer 36, on which the ink chamber 42 is to be formed, is formed on the passivation layer 35. A top surface of the anti-cavitation layer 36 forms a bottom surface of the ink chamber 42 and prevents damage to the heater 33 due to a high pressure caused by bubble collapse in the ink chamber 42. A tantalum thin film is used as the anti-cavitation layer 36.
In this configuration, a barrier layer 40 defining the ink chamber 42 is stacked on the base plate 30 formed of the plurality of material layers stacked on the substrate 31. The barrier layer 40 is formed by coating a photosensitive polymer on the base plate 30 through lamination and patterning a coated resultant. In this case, the thickness of the photosensitive polymer is determined by the height of the ink chamber 42 corresponding to the volume of ink droplets.
A nozzle plate 50, in which the nozzle 52 is formed, is stacked on the barrier layer 40. The nozzle plate 50 is formed of polyimide or nickel (Ni) and is attached to the barrier layer 40 using an adhering property of a photosensitive polymer.
In the thermally driven ink-jet printhead, however, a heater is heated at a high temperature to generate bubbles in ink, such that energy efficiency is low and a remaining energy should be dissipated.
FIG. 2 illustrates a general structure of a piezoelectrically driven ink-jet printhead. Referring to FIG. 2, a reservoir 2, a restrictor 3, a pressure chamber 4, and a nozzle 5, which collectively form an ink passage, are formed in a passage formation plate 1. A piezoelectric actuator 6 is formed on the passage formation plate 1. In operation, the reservoir 2 stores ink flowing from an ink container (not shown), and the restrictor 3 is a path through which ink flows from the reservoir 2 to the pressure chamber 4. The pressure chamber 4 is filled with ink to be ejected, and the volume of the pressure chamber 4 is varied by driving the piezoelectric actuator 6, which causes a variation in pressure for ejection or flow of ink.
The passage formation plate 1 is formed by cutting a plurality of thin plates formed of ceramic, metal, or synthetic resin, forming part of the ink passage, and depositing the plurality of thin plates. The piezoelectric actuator 6 is formed above the pressure chamber 4 and has a structure in which a piezoelectric thin plate and an electrode for applying a voltage to the piezoelectric thin plate are stacked. In this configuration, a portion of the passage formation plate 1 that forms upper walls of the pressure chamber 4 serves as a vibration plate 1a deformed by the piezoelectric actuator 6.
The operation of the piezoelectrically driven ink-jet printhead having the above structure will now be described.
When the vibration plate 1a is deformed by driving the piezoelectric actuator 6, the volume of the pressure chamber 4 is reduced. Subsequently, due to a variation in pressure in the pressure chamber 4 caused by a reduction in the volume of the pressure chamber 4, ink in the pressure chamber 4 is ejected through the nozzle 5. Subsequently, when the vibration plate 1a is restored to an original shape by driving the piezoelectric actuator 6, the volume of the pressure chamber 4 is increased. Due to a variation in pressure caused by an increase in the volume of the pressure chamber 4, ink stored in the reservoir 2 flows into the pressure chamber 4 through the restrictor 3.
FIG. 3 illustrates a structure of a conventional piezoelectrically driven ink-jet printhead. FIG. 4 illustrates a cross-sectional view taken along line IV-IV of FIG. 3.
Referring to FIGS. 3 and 4, the piezoelectrically driven ink-jet printhead is formed by stacking a plurality of thin plates and adhering them to one another. More specifically, a first plate 11, in which a nozzle 11a through which ink is ejected is formed, is disposed in a lowermost portion of a printhead, a second plate 12, in which a reservoir 12a and an ink outlet 12b are formed, is stacked on the first plate 11, and a third plate 13, in which an ink inlet 13a and an ink outlet 13b are formed, is stacked on the second plate 12. A fourth plate 14, in which an ink inlet 14a and an ink outlet 14b are formed, is stacked on the third plate 13, and a fifth plate 15, in which a pressure chamber 15a in communication with the ink inlet 14a and the ink outlet 14b is formed, is stacked on the fourth plate 14. The ink inlets 13a and 14a serve as a path through which ink flows from the reservoir 12a to the pressure chamber 15a. The ink outlets 12b, 13b, and 14b serve as a path through which ink is expelled from the pressure chamber 15a toward the nozzle 11a. A sixth plate 16, which closes an upper portion of the pressure chamber 15a, is stacked on the fifth plate 15. A driving electrode 20, which is a piezoelectric actuator, and a piezoelectric thin film 21 are formed on the sixth plate 16. Thus, the sixth plate 16 serves as a vibration plate that vibrates by the piezoelectric actuator, and the volume of the pressure chamber 15a formed under the sixth plate 16 is varied by deformation of the vibration plate.
In general, the first, second, and third plates 11, 12, and 13 are molded by etching or press-finishing a metallic thin plate, and the fourth, fifth, and sixth plates 14, 15, and 16 are molded by cutting thin-plate-shaped ceramic. In the piezoelectrically driven ink-jet printhead having the above structure, however, in order to obtain an effective displacement of a piezoelectric thin film for ejection of ink droplets, a size of a structure becomes larger. As such, the number of nozzles per unit area is limited. In addition, in order to manufacture the piezoelectrically driven ink-jet printhead, a variety of plates are separately processed using a variety of processing methods, and then, the plates are stacked and adhered to one another. Thus, the plates should be precisely disposed and adhered.
FIGS. 5A and 5B illustrate a structure of another conventional ink-jet printhead.
Referring to FIGS. 5A and 5B, a nozzle 65a is formed on an end of a channel 65 filled with ink 60, and a polymer element 70 is formed around the nozzle 65a. The polymer element 70 may be in a hydrophilic or hydrophobic state according to a temperature value. In this configuration, a heating element 75 for providing temperature control is formed under the polymer element 70.
In the above structure, FIG. 5A illustrates an ink-jet printhead when the polymer element 70 is in a hydrophilic state. In this state, ink 60 contacts the polymer element 70 and stays in contact with the polymer element 70. However, if the heating element 75 increases the temperature of the polymer element 70 to more than a threshold temperature, as shown in FIG. 5B, the polymer element 70 is changed into a hydrophobic state. The threshold temperature is a phase transition temperature of a polymer. When the polymer element 70 is changed into the hydrophobic state, ink 60 is repelled from the polymer element 70. In this state, a predetermined pressure is applied to an ink supply unit 90. Thus, ink 60 is not returned to the ink supply unit 90 and is ejected in droplets through a nozzle 65a onto a sheet of paper 80.
Accordingly, this ink-jet printhead ejects ink droplets using a method of changing a polymer element in a hydrophobic or hydrophilic state depending on a temperature value.
However, unlike the above-described method, the present invention uses a method of ejecting ink droplets by expanding and contracting a volumetric structure sensitive to an external stimulus.